1. Field of the Invention
The present invention relates to a communication interface for the protocol of a layer 1 (electrophysical layer) of the communication using a digital communication network such as an ISDN (Integrated Services Digital Network) or the like.
2. Related Background Art
As shown in FIG. 6, in a standard construction of the ISDN subscribers, a terminal device (hereinafter, referred to as a TE) 50 such as a telephone, facsimile apparatus, or the like is connected to an ISDN network via a network terminal device (hereinafter, referred to as an NT) 51.
The applicant of the present invention has already applied a communication system or communication apparatus using the ISDN disclosed in U.S. patent application Ser. Nos. 151,309 and 258,768.
In FIG. 6, a transmission path between the TE and the NT is defined as an S/T point by the International Telegraph and Telephone Consultative Committee (CCITT). The interface specification called the layer 1 is recommended.
As shown in FIG. 4, in each direction of the transmission between the TE and the NT, a bit train is constructed by a frame consisting of 48 bits. The frame construction in the case of transmitting from the TE to the NT is different from that in the case of transmitting from the NT to the TE.
In the case where the TE transmits a bit train of a frame 41 to the NT or in the case where the TE receives a bit train of a frame 40 from the NT, the TE extracts and reproduces a clock signal from the bit train received from the NT and transmits or receives the bit train by using the timing of the clock signal. On the other hand, in the case where the NT transmits the frame 40 to the TE, the NT transmits the bit train by using a clock signal from the ISDN.
In the case where the NT receives the frame 41 from the TE, the NT extracts and reproduces a clock signal from the bit train received from the TE and receives the bit train from the TE by using the clock signal.
On the other hand, in the idling state, that is, in the use waiting state, the transmission path between the TE and the NT is set into the stop state to reduce the electric power consumption. Therefore, in order to correctly communicate, the layer 1 in the stop state is set into the start state. On the contrary, when the communication is completed, the layer 1 in the start state is set into the stop state.
FIG. 7 shows start and stop procedures in such a case. FIG. 7A shows the start procedure in the case of performing a start request from the TE side. INF01 denotes a signal to indicate the start request to the NT. INF02 represents a signal which is necessary for the TE to perform the synchronization. That is, the INF02 is the signal in which the channel bits (B.sub.1, B.sub.2, D) are 0 in the frame 40. On the basis of the INF02 signal, the TE extracts a clock signal which is necessary to transmit data. INF03 denotes a signal indicating that the synchronization was obtained for the NT. INF04 is a signal to indicate the transmission of data to the TE. FIG. 7B shows the start procedure in the case where the start request is executed from the NT to the TE in response to the terminating from the ISDN network side. FIG. 7C shows the stop procedure from the network side. As mentioned above, in the S/T point interface of the ISDN, the bit construction of the frame differs dependence on the transmitting direction. In addition, the control functions of the NT and TE also differ. Therefore, in the control of the conventional ISDN layer 1 , the control functions are respectively independently distinguished for the TE and NT.
Therefore, for the S/T point, an interface for the NT layer 1 must be provided for the NT, while another interface for the TE layer 1 must be also provided for the TE.
Further, in the interfaces for the TE layer 1 and NT layer 1 , since the clock signals which are used upon transmission and reception differ, there is a drawback such that even when two TE terminals are simply connected, the two-way connection test (maintenance and inspection services such as failure analysis and the like) of each TE terminal cannot be executed.
Therefore, even in the case of executing the two-way connection test, the TE terminal must be also connected to the ISDN via the NT in a manner similar to the case where the actual data communication is performed, so that there is a drawback such that the two-way connection test of the TE terminal cannot be easily executed.